Plastic insulating materials are used for example in the cable industry. Such plastic materials have heretofore been cured by adding a crosslinking agent which, under the influence of high temperature, breaks down to form radicals which initiate the crosslinking. Because of the poor heat conductivity of the insulation, a long time is required to bring the cable insulation to the proper high temperature and this requires a long length of cable to hang unsupported in the heating zone in order to avoid deforming the insulation. This necessitates a high equipment expense. Moreover, this process has the disadvantage that the crosslinking is not uniform throughout the insulation as the heat penetrates the insulation from the outside and hence the crosslinking is not initiated simultaneously throughout the entire insulation cross section. This disadvantage can unfavorably influence the electrical properties of the insulation.
In German Pat. No. 26 11 349 there is disclosed a process of crosslinking plastic material in which the crosslinking process of a plastic material which is not excitable in an alternating electric field is initiated through the reaction of a composition which is unstable in an alternating electric field. In this process the decomposition of the crosslinking agent into radicals is effected not through warming the entire mass but through the use of a radical source which by reason of its special composition is excitable by the energy of an alternating current field and can thereby initiate the crosslinking process in that it, for example, decomposes into radical components which effect the crosslinking.
Such crosslinking agents are for example certain peroxides or systems of radical sources with substances which transfer the energy from an alternating current field to the radical donor. The substances or mixtures heretofore used have the expected characteristic that after completion of the crosslinking process they still retain a large part of their activity in an alternating current field. Thus after the crosslinking process, the plastic material has the characteristic of being inductive in an alternating current field. For example, the crosslinking agent may comprise a mixture of a peroxide such as di-t-butylperoxide which is not excitable with an excitable additive such as tri-allyloxi-s-triazine, special carbon black or halogenated hydrocarbon (chloronated paraffin). The plastic material produced by this method has poor insulation characteristics.